Add new api function mariadb_stmt_fetch_fields

To get metadata information for the binary protocol (prepared statements), a resultset handle must be allocated via mysql_stmt_result_metadata() which contains a pointer to stmt-> fields. Afterwards this handle must be released via mysql_free_result().

Instead of allocating and freeing memory it would be much simpler to have an additional API function mariadb_stmt_fetch_fields() which just returns stmt->fields:

MYSQL_FIELD STDCALL *mariadb_stmt_fetch_fields(MYSQL_STMT *stmt)

{

  if (stmt)

    return stmt->fields;

  return NULL;

}